The present invention relates generally to the manufacture of high voltage cable of the type comprising a central conductor covered by a conductor shield, insulation and an insulation shield; and specifically to an apparatus for and method of continuously detecting skips, protrusions, protuberances and other defects in cable conductor shield.
Conductor shield is employed to provide a uniform electric field at the interface between the cable conductor and insulation. Points of discontinuity in the conductor shield, such as voids or protuberances in the shield become points of high voltage stress on the insulator, leading to untimely cable failure. Furthermore, the degree of stress resulting from a conductor shield discontinuity is largely determined by the geometry of the discontinuity. A small, very pointed protuberance in the conductor shield will be more deleterious to cable performance than will a relatively larger rounded protuberance.
To prevent premature cable failure, detection of skips, protrusions, protuberances and other defects in the shielding of cable conductors during cable manufacture is necessary.
Widespread underground use of high voltage cable of the type comprising a central shielded conductor, insulation and insulation shield began in the 1960's. In the early 1970's these cables began to fail. A major cause of failure was the phenomenon known as electrochemical treeing, and investigation has shown that conductor shield defects are the source of a substantial number of electrochemical trees. Since that time the cable industry has searched for methods of reducing cable failures by identifying and reducing conductor shield defects.
U.S. Pat. No. 3,841,810, discloses one of first attempts to detect conductor shield imperfections, namely, a contact system which measures resistance between the conductor and a portion of the insulation extruder. Several major problems are inherent in that system. For example, the system cannot determine the severity of defects, that is, it cannot distinguish between harmless gradual circumferential changes and sudden changes which are harmful. Since it is a contact system, the component contacting the conductor shield must be changed each time the conductor dimension changes. In addition, the system is easily fouled by contaminants such as die bleed accumulations which cannot pass through the detection component.
In April 1979, during the SEVENTH IEEE POWER ENGINEERING SOCIETY TRANSMISSION AND DISTRIBUTION CONFERENCE AND EXPOSITION, Phelps Dodge Co. reported on its efforts in the field. The Phelps Dodge process includes a protrusion limiter which electronically examines the extrusion compound for metal particles and rejects any contaminated pellets before they reach the extruder. An optical skip detector comprising numerous optical sensors detects skips by recognizing change in reflected light. This method is inefficient because examination of the extrusion compound can be performed much more easily and less expensively by a sample method and because an optical skip detector is limited to detection of apparent defects and cannot detect hidden defects.
Other methods for detecting conductor shield defects prior to the development of this invention include post-manufacture corona tests on finished cable and capacitance tests on the conductor prior to application of the insulation shielding.